Conformal Coating Protects Medical Electronics

To apply traditional coatings to printed circuit board (PCB) surfaces, manufacturers must mask components that should not be coated--a processing operation that can increase costs. In an effort to circumvent this laborious step, Semblant (Chandler, AZ) has developed Plasma Shield, a conformal coating that can be used to coat electronic components for such medical devices as hearing aids and hospital equipment.

"One of the things our coating enables us to do is to reduce the number of manufacturing steps required to apply a conformal coating on electronics," remarks Steve McClure, vice president, worldwide sales and marketing, at Semblant. "With traditional conformal coats, one of the things that people often have to do is mask the parts of the PCB with tape that can't be coated. Connectors are a common example. If you coat the connector when you make the mating, you don't get a good contact." Measuring only 1 µm thick, Semblant's conformal coating, in contrast, enables manufacturers to achieve a good connection.

A fluoropolymer material that is very much like Teflon, the coating is deposited in a vacuum chamber. When the PCB is placed in the chamber between electrodes, a process gas is injected that energizes the electrodes, converting it into a plasma or solid that adheres to the electronics. This process is performed at room temperature, and there is no cure time. The result, according to McClure, is a very pervasive and continuous coating over the electronics. Used to prevent residual corrosion, the coating protects electronics from moisture or from such contaminants as earwax, sulfur, pollutant gases, or salts. All of these contaminants, McClure adds, can harm electronics.

"If you need to rework a component on a PCB after our coating has been applied, you can remove it, replace it, and recoat the board," McClure says. "With other coating processes, you have to scrape off the coating, replace the component, and then touch up the coating around the affected area. But this creates a seam, producing an area that is susceptible to moisture. One of the fundamental attributes of Semblant's technology is that it does not produce seams, according to McClure.

Another attribute of the coating is its ability to reduce the incidence of tin whiskers--defects that can cause shorts and device failures. In a series of preliminary comparative studies into tin whisker growth, Semblant's aerospace customers have determined that the new coating can mitigate tin whiskers as well as or better than traditional conformal coatings, McClure comments. "The coating doesn't stop whiskers from growing--no conformal coats can stop whiskers from growing. But what it does do is inhibit the growth of whiskers. Gnarly, crinkly whiskers can snap off and become debris within the electronics assembly. In our case, we avoid gnarly whiskers."

Another challenge facing manufacturers is to create watertight plastic enclosures to prevent contamination from the isopropyl alcohol (IPA) wipes that are used to clean hospital equipment. However, such enclosures add complexity and expense to the assembly process. Because Semblant's coating protects the electronics directly, the cost of producing plastic enclosures can be reduced, lowering the overall costs associated with designing and manufacturing hospital equipment. "In our case," McClure says, "it doesn't matter if the alcohol gets onto our coating because it won't get through it and into the electronics."

For medical device subassemblies that are produced in a different location from the final assembly, contamination control becomes a crucial issue. In such cases, manufacturers have an interest in avoiding such contaminants as corrosive gases or environmental pollutants that can affect device components while in transit. "Because medical device performance is so critical, reliability assurance is essential," McClure states. "A conformal coat during the interim assembly process protects electronic components before they are assembled into the final product."

Bob Michaels is managing editor of Medical Product Manufacturing News.